Orifice Restrictors - Design Guidelines

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-OE-037

ORIFICE RESTRICTORS:
Design Guide Lines

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CONTENT

ORIFICE RESTRICTOR: DESIGN GUIDELINES
SPECIFICATION OF FUNCTION
DESCRIPTION OF FLUID
INLET FLUID PHASE(S)
FLOWRATE
UPSTREAM PRESSURE
DOWNSTREAM PRESSURE
OPERATING TEMPERATURE
TEMPERATURE EXTREMES: MINIMUM
TEMPERATURE EXTREMES: MAXIMUM
UPSTREAM DENSITY
UPSTREAM COMPRESSIBILITY FACTOR.
IS RESTRICTOR PART OF SAFETY SYSTEM?
LINE SIZE
LINE REFERENCE
CALCULATED ORIFICE DIAMETER

OTHER DESIGN INFORMATION


ORIFICE RESTRICTORS:

DESIGN GUIDELINES

SPECIFICATION OF FUNCTION
The function is a brief description of the purpose of the orifice restrictor
and is used as a descriptive title for it. The information may be supplemented by
an ELD (Engineering Line Diagram) grid location or Line Reference or Equipment
No. to identify the location of the restrictor.
DESCRIPTION OF FLUID
The name of the fluid should be given together with its concentration or
composition where the latter are important from the point of view of corrosion or
safety. The choice of materials of construction of the orifice restrictor should be
compatible with the composition of the fluid under normal operation, but should
also take into consideration abnormal compositions of the fluid due to
maloperation of the process or the operating system, as well as start up and shut
down conditions. It should also be used to describe mixtures of fluids with
unusual or vastly differing chemical or physical properties. These may be
expanded under 'Other Design Information'.
INLET FLUID PHASE(S)
State the fluid phase or phases at the restrictor inlet. If the fluid is 'dirty', i.e. it
contains solid particles which could deposit on or erode the restrictor, and then
the fluid should be specified as 'dirty'. If the fluid is a saturated gas at the
restrictor inlet it should be described as 'wet'. Examples are Gas/dirty, Gas/wet,
Solids/liquid (slurry), Liquid/dirty, etc..
FLOWRATE
If the fluid is steam, give the mass flowrate in kg/h; for liquids and gases give the
volumetric flowrate in m3/h. Gas flowrates should preferably be at 0 °C and 1.013
bar ABSOLUTE (bar abs.). Only one of the fields for flowrate should be filled in; a
facility exists for calculating the other from the data provided.
UPSTREAM PRESSURE
Give the upstream pressure of the fluid in bar abs.. The facility to use bar
GAUGE (bar g.) is also provided.


DOWNSTREAM PRESSURE
Give the pressure downstream of the restrictor in bar abs. or bar g. (same units
as for Line 12). The difference in pressure between the upstream and
downstream pressures should be used to size the restrictor orifice diameter so
that the pressure drop is lost across the restrictor.
OPERATING TEMPERATURE
Give the temperature at the restrictor inlet in °C under normal flowsheet
operating conditions.
TEMPERATURE EXTREMES: MINIMUM
Give the lowest temperature extreme in °C to which the orifice restrictor may be
exposed. This information is used to select the correct material of construction of
the restrictor plate.
TEMPERATURE EXTREMES: MAXIMUM
Give the highest temperature extreme in °C to which the orifice restrictor may be
exposed. This information is used to select the correct material of construction of
the restrictor plate.
UPSTREAM DENSITY
Give the actual density of the fluid at the normal upstream conditions in kg/m3.
UPSTREAM COMPRESSIBILITY FACTOR.
For gases only, give the compressibility factor, Z, at the normal upstream
conditions.
0°C/760 mm Compressibility Factor. For gases only, give the compressibility
factor, Z, at 0 °C and 760 mm Hg (1.013 bar abs.).
IS RESTRICTOR PART OF SAFETY SYSTEM?
If the restrictor is not part of a safety system, answer "NO". If it is, answer "YES"
or give the reference to the safety system.
A restrictor which is part of a safety system requires registration.

LINE SIZE
For pipe lines, the nominal pipe size should normally be taken from the ELD. The
units should be either mm NOMINAL or inches NOMINAL and not exact
conversions.
LINE REFERENCE
In the case of new plants, the Line Reference provides the key to information on
line specification and material of construction requirements. In the case of
existing plants, the flange rating and material of construction should be specified.
The data is normally taken from the ELD.
CALCULATED ORIFICE DIAMETER
Give the calculated orifice diameter in mm. This information, normally calculated
by the process engineer, is used to check that the correct orifice is specified and
installed.

OTHER DESIGN INFORMATION
Special Features
If any special features affect the choice of the orifice restrictor, they should be
recorded in the 'Other Design Information' section.
The following is a list of topics which are likely to call for comment:
(a)

Brief description of any unusual or special factors affecting the selection of
data entered on the orifice restrictor data sheet.

(b)

Any special reasons for the choice of restrictor.

(c)

Any special reasons for the choice of materials of construction.

(d)

Any other information considered relevant by the process engineer.


Additional Data
There may be occasions when information other than that specifically requested
by the client is needed.
The following list is intended to give examples and indicate where it is especially
important for the process and control & electrical engineers to liaise closely.
(a)

Requirement for sterile conditions.

(b)

Where the restrictor is expected to operate under widely differing
operating conditions, e.g. cyclic operations.

(c)

Conditions of pressure surge (e.g. water hammer or start up of a steam
main) which could cause deflection of the orifice plate unless an adequate
thickness is specified by the control electrical engineer.
A computer program exists to calculate the required thickness. If the plate
thickness is specified as a result of such calculation, it should be quoted.


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